Glass capillary tubes with an inner diameter of less than 1 mm are often used to handle small, sub-microliter fluid volumes in chemistry and biology. With the advent of advanced gene sequencing research, automated systems are used to prepare a large number of small, sub-microliter samples in a relatively short period of time (e.g. 5,000 samples in eight hours). For small fluid volumes, a long visible fluid sample (also called a slug) is aspirated into a capillary tube encased within dead air volume. The aspirated fluid within the dead air volume is more easily handled, observed, and controlled by a researcher/technician. Often, the capillary tubes themselves become individual reaction chambers in which solution containing DNA, or other sampled material is combined with a plurality of reagents. The aspirated fluid must be mixed and the air bubbles removed before the resulting mixture is ready to be incubated or thermal cycled, depending on the reaction of interest.
Traditionally, small volumes have been mixed in biochemistry laboratories through hand-pipetting one sample into a container holding another mixture. The turbulence as the fluid enters and leaves the nozzle tip of the hand-pipette and the large exposed surface area between the fluids allows mixing to progress quickly. However, this method is labor intensive, not conducive to automation, and risks losing sample to the outer surface of the capillary tube from surface tension.
In automated processes, the known mixing options are limited. The mixing time in automated systems needs to be relatively quick, for example, mixing one microliter water solution within three seconds. Previously-known mixing devices include a precision linear actuator, which is coupled to the capillary tube by an O-ring that is used to move the fluid. However, precision linear actuators are expensive and bulky. Additionally, the speeds obtained through precision linear actuators are limited to approximately 200 mm per second or mm/sec. Some commercial units can move over 1 mm/sec., but the response time of a particular fluid slug can be limiting. Another limitation with piston-drive actuators is that the system can become overheated due to friction between the piston and the O-ring. This friction can negatively affect system reliability and reduce the allowable mixing envelope.
Another known mixing method within an automated system is disclosed in Mochida, U.S. Pat. No. 4,960,566, granted Oct. 2, 1990 and entitled, "Chemical Reaction Apparatus." Mochida discloses the use of capillary action to fill the entire capillary with fluid. Mochida discloses injecting a wash solution and expelling it out to achieve mixing. This process requires more time to cycle through the automated system disclosed in Mochida. Also, the system does not allow for the visibility of the fluid slug, which is desirable because of the ability to easily handle, observe and control the slug.
It is an object of the present invention to completely mix sub-microliter volumes that are aspirated between dead air volumes within the chamber of a capillary tube, and that such device be amenable to incorporation into a high-speed automation process.